The present invention is directed toward curing methods for elastomeric articles. Specifically, the present invention discloses a method of producing faster curing rubber and the application of the faster curing rubber in articles for bonding.
One of the many conflicting requirements of the rubber industry is for compounds that have a short cure time and a long scorch time. The conflict arises because scorch time cannot be changed independently of cure time; the times increase or decrease together. Conventionally, productive compounds, i.e. compounds that are capable of curing, are made in a Banbury mixer that generates beat in the compound during mixing. The compounds are then stored and subjected to further heat history during shaping of the compound by extrusion or calendering. The extruded or calendered article may be stored prior to application of the article in a larger green rubber article. The formed green article may also be further stored until curing. This entire process requires a certain minimum scorch time.
Reductions in cure times are proportional to increases in cure temperatures; however, reduced cure times can no longer be achieved by merely increasing the temperature of a curing press. Cure presses are conventionally run at the maximum temperature permitted to avoid overcure of the outside of the article while not exceeding any component cure limitations.
To obtain a maximum scorch time for rubbers it is also known to not mix the cure package into the rubber compound until the rubber is to be used. The green rubber, absent a mixed cure package, may be stored indefinitely until it is needed for article manufacturing. When the green rubber is needed, the rubber is mixed in a Banbury with the appropriate cure package.
Another known alternative to extend the shelf life of the green rubber is to split the cure package. EP 496,202 discloses a two component system wherein the curatives are split between the two components. The two components must be masticated in a conventional mixer prior to use to achieve a thorough blend of the curatives and gain a productive compound.
U.S. Pat. No. 5,866,265 discloses a way to prevent scorch during extrusion of rubber microlayer compounds comprised of alternating layers of different rubber composition. The cure package is split in any desired manner between the two different rubber compositions, the different compositions are kept separated in different barrels of the co-extruder until they are layered in the extruder die and the curatives migrate into the adjacent layers.
In order to use faster curing compounds in manufacturing rubber articles, the heat history imposed on productive compounds must be reduced. The present invention is directed toward overcoming the limitations of the prior art and producing even faster curing rubber compounds.
The present invention is directed to an improved method of bonding at least two cured or uncured elastomeric layers. The method comprising placing an uncured elastomeric component between the two elastomeric layers, and curing the elastomeric component to bond the elastomeric layers together. The improvement is characterized by forming the uncured elastomeric component by preparing two non-productive elastomer compounds, wherein each non-productive compound is prepared with a co-reacting agent of a co-reacting cure package not added to the other non-productive compound, and layering the non-productive elastomer compounds in alternating layers with a thickness relative to the diffusion rate of the co-reacting agents in each non-productive elastomeric layer to effect diffusion of the co-reacting cure agents through the adjacent layers.
In one aspect of the inventive method, the elastomeric achieves ninety percent cure, at a cure temperature of 120xc2x0 C., in less than 30 minutes.
In another aspect of the disclosed method, the two non-productive compounds forming the uncured elastomeric component are prepared with identical compositions except for the co-reacting cure agents in each compound.
In another aspect of the disclosed method, the first non-productive compound forming the uncured elastomeric compound is prepared with an absence of any accelerators found in the second non-productive compound and the second non-productive compound is prepared with an absence of any sulfur vulcanizing agent found in the first non-productive compound.
In yet another aspect of the disclosed method, the fist non-productive compound is prepared with 1 to 5 phr zinc oxide and 0 phr sulfur vulcanizing agent and the second non-productive compound is prepared with 0 phr zinc oxide and 0.2 to 8 phr sulfur vulcanizing agent.
In one aspect of the disclosed method of improved bonding, the co-reacting agents of the cure package are selected to produce an ultra fast cure of the elastomeric component.
In another aspect of the disclosed boding method, the adjacent layers of the two non-productives are formed with a thickness equal or less than 2 mm. The layer thickness of the adjacent layers may be identical or may differ, depending upon the desired cure characteristics or the end characteristics of the elastomeric component.
In one aspect of the disclosed method, the two non-productive compounds forming the uncured elastomeric component may be stored for any period of time prior to layering to form the uncured component. In another aspect of the inventive method, the layered component may be stored for any period of time prior to curing the elastomer.
In an aspect of the disclosed method of bonding at least two cured or uncured elastomeric layers, the cured or uncured elastomeric layers are two different components of an article selected from the group consisting of a passenger tire, an extended mobility tire, a truck tire, an earth mover tire, a retreaded tire, a belting, an airspring sleeve, or a rubber track.
In another aspect of the disclosed method, the elastomeric layers to be bonded by means of the uncured elastomeric component are two different tire components. In another aspect of the disclosed method, the two different tire components are a prepared tire casing and a pre-cured tire tread.
In one aspect of the disclosed method of bonding a tire casing and a pre-cured tire tread together by means of the uncured elastomeric component, the uncured elastomeric component is prepared by layering the two non-productive compounds immediately prior to inserting the elastomeric component between the tire casing and the ire tread.
In a further aspect of the disclosed method of retreading a tire, the tire tread is preheated prior to placing the tread on the uncured elastomeric component. In another further aspect of the disclosed retreading method, the layered uncured elastomeric component is cured at room temperature.